Technical Field
The present disclosure generally relates to a semiconductor device, and, more particularly, to a three-dimensional memory device and a method for manufacturing the same.
Discussion of Related Arts
A semiconductor device may include a memory device for storing data therein. When a memory device is implemented as a NAND flash memory device, it may include a plurality of strings, each string comprising a plurality of memory cells connected in series. For achieving a high integration of the semiconductor device, a three-dimensional memory device has been suggested which has a series connection of three-dimensional arrangements of memory cells on a memory string basis.
For example, two vertical arrangements of the memory cells are coupled to each other via a pipe transistor to form a single memory string.
FIGS. 1A and 1B illustrate threshold voltage shift characteristics of a memory device such as a NAND flash memory device due to multiple iterations of program and erase operations. More particularly, FIGS. 1A and 1B illustrate a distribution of a threshold voltage of a pipe transistor and of memory cells, respectively.
Referring to FIG. 1A, it is seen that, as the number of repetitions of the program and erase operations, in other words, the number of erase/write (EW) cycles increase, the threshold voltage distribution of the pipe transistor shifts to the right, i.e., to higher voltages. When the threshold voltage distribution of the pipe transistor increases, a program disturb characteristic of the memory cells may deteriorate as shown in FIG. 1B.